IC package pressure release apparatus and method

ABSTRACT

An apparatus and method for releasing pressure existing within a package comprising a substrate to which a die is attached to provide electrical connections between the die and the exterior of the package, a lid, and sealant disposed between the substrate and the lid in a pattern with at least one break in the pattern.

FIELD OF THE INVENTION

The present invention is related to an apparatus and method forincreasing the reliability and performance of an electronic deviceenclosed within a flip-chip package.

ART BACKGROUND

Commonly available electronic devices exist in the form of a thin sheetof semiconductor material, or die, with electronic circuitry disposedthereon by way of various photolithographic processes. To protect thecircuitry from damage, the die is often enclosed in a package designedto facilitate the attachment of the electronic device to a printedcircuit board.

Improvements in electrical signal characteristics, as well as increasedflexibility in power consumption and die size, have been realizedthrough the use of packages such as the “flip-chip” or “controlledcollapsed chip connection” (C4) package which typically integrates a lidmade of aluminum or other thermally conductive material that serves as aheatsink (also known as a “heat spreader”), and a substrate made of anorganic compound that incorporates electrically conductive materialsused to provide electrical connections between the die and acircuitboard to which the package is attached.

FIG. 1 is a cross-sectional of a prior art package of the ball gridarray type. The exterior of package 100 is comprised of lid 110 (alsoknown as an integrated heat spreader, or IHS), substrate 112 and sealant114 disposed between lid 110 and substrate 112. Thermal attach 116provides a thermally conductive connection between die 130 and lid 110,which serves to aid in cooling die 130 during normal operation of thecircuitry disposed on die 130 by emitting heat conducted from die 130through thermal attach 116 to the ambient air surrounding the exteriorof lid 110. Underfill 120 and solder balls 122 (also known as C4 bumps)attach die 130 to substrate 112, with solder balls 122 providingelectrical connections between die 130 and substrate 112. Substrate 112is a printed circuit board with conductors forming electricalconnections between solder balls 122 and solder balls 124. Solder balls124 form electrical connections between substrate 112 and circuitboard140 when package 100 is attached to circuitboard 140, and thereby, formelectrical connections between die 130 and circuitboard 140.

FIG. 2 is a top side view of the same prior art package depicted in FIG.1, with the items of package 200 of FIG. 2 being labeled with 2xxnumbers that correspond to the 1xx numbers of the labeled items ofpackage 100 of FIG. 1. What would have been a lid and thermal attachcorresponding to lid 110 and thermal attach 116, respectively, of FIG. 1have been removed from the package depicted in FIG. 2 to allow therelative position of other items under the lid to be seen. As shown inFIG. 2, the exterior of package 200 is comprised of substrate 212 andsealant 214. Die 230 is substantially centered relative to substrate212, and is attached to substrate 212 via underfill 220 shown protrudingfrom underneath and just beyond the edges of die 230. Sealant 214 isdisposed to correspond to where the lid (not shown) would meet withsubstrate 212, so as to bond the lid to substrate 212. As shown, sealant214 is disposed in a manner forming an unbroken line that surrounds die230.

A drawback to disposing sealant 214 in an unbroken line is that acomplete seal is formed between the lid and substrate 212 that canprevent differences in pressure between the interior of package 200 andthe ambient air surrounding the exterior of package 200 from beingequalized. This becomes especially significant where substrate 212 ismade of organic or other material that is susceptible to absorbingmoisture from the ambient air surrounding package 200, and thenreleasing moisture into the interior of package 200. During thermaltesting or normal use, when the temperature of die 230 increases,moisture present within the interior of package 200 become steam,causing an increase in pressure within package 200, and tending to pushthe lid and substrate 212 apart. As can best be appreciated by reviewingFIG. 1, the pushing apart of lid 110 and substrate 112 tends to separatelid 110 from die 130, thereby reducing the pressure normally exerted bylid 110 to squeeze thermal attach 116 against die 130, and therebyreducing the effectiveness of thermal attach 116 in conducting heat awayfrom die 130. This increase of pressure can also cause substrate 112 tobow outward, giving substrate 112 a curved shape that exerts stress onthe corners of die 130 which may cause thin film cracking and/or thinfilm delamination leading to electrical failure and/or decreasedlifespan of the circuitry disposed on the surface of the die.

Still another drawback arising from the presence of moisture beingconverted to steam under pressure, is that steam could be forced betweeneither thermal attach 116 and die 130, or between thermal attach 116 andlid 110. This reduces the contact between thermal attach 116 and eitherlid 110 or die 130, and thereby also reduces the effectiveness ofthermal attach 116 in conducting heat away from die 130 and towards lid110. Furthermore, depending on the composition of thermal attach 116,steam may also penetrate the thermal attach material, itself, therebyforming gaps within thermal attach 116 which would block thetransmission of heat from die 130 through thermal attach 116.

In addition to undesirable effects resulting from a buildup of pressureand the presence of moisture, there are drawbacks arising from the useof an unbroken line of sealant as a result of differences in the ratesof expansion and contraction between the lid, the substrate and the die.Referring once again to FIG. 1, as temperature increases, both lid 110and substrate 112 tend to expand more rapidly than die 130, and atdifferent rates relative to each other, depending on the particularmaterials used in making lid 110 and substrate 112. An unbroken line ofsealant restricts relative movement of lid 110 and substrate 112 morethan sealant with a pattern of breaks would, and as a result, either lid110 or substrate 112 can be caused to bow outwardly. As earlierdescribed, this bowing out effect can either reduce the effectiveness ofthermal attach 116 in conducting heat away from die 130, or can causecracking and/or delamination in die 130.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects, features, and advantages of the present invention will beapparent to one skilled in the art in view of the following detaileddescription in which:

FIG. 1 is a cross-sectional view of a prior art ball grid array package.

FIG. 2 is a top side view of a prior art ball grid array package withsome portions of the package removed.

FIG. 3 is a top side view of one embodiment of the present inventionwith some portions of the package removed.

FIG. 4 is a top side view of another embodiment of the present inventionwith some portions of the package removed.

DETAILED DESCRIPTION

In the following description, for purposes of explanation, numerousdetails are set forth in order to provide a thorough understanding ofthe present invention. However, it will be apparent to one skilled inthe art that these specific details are not required in order topractice the present invention.

The present invention concerns “flip-chip” or “controlled collapsed chipconnection” (C4) packages for integrated circuits where an organicsubstrate is used. Such a package is also known as an “organic land gridarray” or OLGA. Particularly, the present invention concerns the use ofsuch packages to hold particularly large dies. However, as those skilledin the art will appreciate, the present invention is also applicablewhere other porous substrate materials are used, or where smaller diesare used upon which is disposed circuitry that generates considerableamounts of heat. The present invention is also applicable where thepackage is to be used in high heat environments.

FIG. 3 a is a top side view similar to that of FIG. 2, and FIG. 3 b is asectional view similar to that of FIG. 1, but depicting an embodiment ofthe present invention along the side view depicted by FIG. 3 c. Items ofpackage 300 of FIGS. 3 a–c being labeled with 3xx numbers thatcorrespond to the 2xx numbers of the labeled items of package 200 ofFIG. 2. Just as with FIG. 2, what would have been a lid and thermalattach corresponding to lid 110 and thermal attach 116, respectively, ofFIG. 1 have been removed from the package depicted in FIG. 3 a to allowthe relative position of other items under the lid to be seen. Asdepicted in FIGS. 3 a–3 c, the exterior of package 300 is comprised ofsubstrate 312 and sealant segments 314 a through 314 d. Also as shown,die 330 is substantially centered relative to substrate 312, and isattached to substrate 312 via underfill 320 and solder balls 322 (alsoknown as C4 bumps, like solder balls 122 of FIG. 1) with underfill 320shown protruding from underneath and just beyond the edges of die 330.Similarly to solder balls 122 of FIG. 1, solder balls 322 provideelectrical connections between die 330 and substrate 312. Also similarlyto FIG. 1, substrate 312 is a printed circuit board with conductors 313forming electrical connections between solder balls 322 and solder balls324.

Like sealant 214 of FIG. 2, the sealant used in the embodiment of FIGS.3 a–c is disposed to correspond to where lid 310 meets with substrate312, so as to bond lid 310 to substrate 312. However, unlike sealant 214of FIG. 2, the sealant used in this embodiment does not form an unbrokenline that surrounds die 330. Instead, the sealant is disposed in foursealant segments 314 a–d, leaving breaks between the segments throughwhich pressure that would otherwise build up within the interior ofpackage 300 as a result of moisture being turned to steam when thetemperature within the package increases may be released.

Referring variously to both FIGS. 1 and 3 a–c, by allowing this releaseof pressure, lid 310 is not caused to be pushed away relative tosubstrate 312. As a result, the pressure exerted by lid 310 on thermalattach 316 that conducts heat away from die 330 to lid 310 is reduced,and the effectiveness of the thermal attach in conducting away such heatis preserved.

Also, by allowing this release in pressure, substrate 112/312 is notcaused to bow outward, thereby causing substrate 112/312 to becomecurved, and potentially causing cracks to form in die 130/330 in thevicinity of the corners.

Furthermore, disposing sealant 314 a–d in a pattern with breaks resultsin the sealant forming a less rigid connection between lid 110 andsubstrate 112/312, thereby allowing a degree of relative movementbetween lid 110 and substrate 112/312. Allowing some degree of relativemovement allows lid 110 and substrate 112/312 to expand and contract atdifferent rates relative to each other in response to changes intemperature, thereby reducing the tendency for lid 110 or substrate112/312 to bow outwardly. This in turn, serves to reduce the earlierdescribed loss in effectiveness of thermal attach 116 and/or the earlierdescribed cracking or delamination in die 130/330.

In one embodiment, the sealant is disposed in multiple segments,resulting in multiple breaks. In another embodiment, the sealant isdisposed in a single segment, but not forming a complete loopsurrounding die 330, thereby resulting in a single break. In oneembodiment, the breaks comprise at least 10% of the total length of whatwould otherwise be a complete loop of sealant, while in anotherembodiment, the breaks comprise less than 10% of that length. In stillanother embodiment, the sealant is disposed in a pattern of small spotsor dots of sealant leaving breaks between the spots of sealant, ratherthan segments defining breaks.

In one embodiment, the sealant is comprised of a relatively flexiblematerial permitting substrate 312 and lid 310 to move relative to eachother with a high degree of freedom. In another embodiment, the sealantis comprised of a more rigid material limiting the relative movement ofsubstrate 312 and lid 310.

In one embodiment package 300 would be formed by first attaching die 330to substrate 312, thereby forming physical and electrical connectionsbetween die 330 and substrate 312. The sealant would then be disposed onsubstrate 312 in the four segments 314 a–d, where lid 310 is to beattached to substrate 312. Thermal attach 316 would then be disposed ondie 330, where lid 310 is to be attached to die 330. Then lid 310 wouldbe attached to substrate 312 (via sealant 314 a–d) and die 330 (viathermal attach 316). As those skilled in the art will understand, theorder in which these occur may be changed without departing from thespirit of the invention.

In one embodiment, package assembly equipment originally designed todispose sealant in an unbroken line is modified to dispose the sealantas sealant segments 314 a–d. In another embodiment, new package assemblyequipment specifically designed to dispose the sealant as sealantsegments 314 a–d is used.

In one embodiment, substrate 312 is comprised of an organic material,and this organic material may be porous such that it is susceptible toabsorbing moisture from the ambient air external to package 300. Inanother embodiment, substrate 312 is comprised of a non-organicmaterial, but a material that still suffers the drawback of absorbingmoisture. In one embodiment, the package and/or the die within thepackage is tested by applying heat to the exterior of the package byexposing the package to steam at high pressure.

In one embodiment, lid 310 has one or more vent-holes, such as vent-hole311, formed therethrough, that could also serve to permit the release ofpressure within the interior of package 300, but which may be blocked asa result of the manner in which package 300 is installed, either fortesting or for normal use. In another embodiment, the lid has novent-holes formed therethrough, and the breaks between sealant segments314 a–314 d are the sole means provided for release of pressure.

In one embodiment, thermal attach 316 is a thermal grease, while inanother embodiment, thermal attach 316 is a thermal adhesive.

In one embodiment, lid 310 is made of material capable of conductingheat away from die 330, such as copper or aluminum. In one embodiment,lid 310 releases the heat conducted away from die 330 to ambient airsurrounding package 300. In another embodiment, lid 310 conducts theheat to other thermally conductive apparatus with which lid 310 is incontact, such as a heatsink (or heat spreader), heat pipe, or thermalelectric cooler.

In one embodiment, package 300 is a ball grid array package wheresubstrate 312 is attached to a circuitboard (not shown) by way ofsolderballs corresponding generally to solder balls 124 of FIG. 1. Inanother embodiment, package 300 is a pin grid array package wheresubstrate 312 is attached to a circuitboard by way of pins, eitherindirectly through a socket attached to the circuitboard, or directlythrough holes drilled through the circuitboard.

In one embodiment, circuitry comprising the core logic (e.g., randomaccess memory controller, bus interface, I/O device interface, ortimers) of a microcomputer system is disposed on a surface of die 330.In another embodiment, circuitry comprising a central processing unit ofa microcomputer system is disposed on a surface of die 330.

FIG. 4 a is a top side view similar to that of FIG. 3 a, and alsodepicts an embodiment of the present invention. FIGS. 4 a–c are providedto show an alternate pattern by which the sealant may be disposed tocorrespond to where lid 410 would meet with substrate 412, so as to bondlid 410 to substrate 412. Although the patterns in both FIGS. 3 a and 4a are both substantially rectangular, the breaks in the sealant inpackage 400 are at the corners of the substantially rectangular pattern,instead of being at the sides as in the case of package 300. Items ofpackage 400 of FIGS. 4 a–c being labeled with 4xx numbers thatcorrespond to the 3xx numbers of the labeled items of package 300 ofFIGS. 3 a–3 c. Just as with FIG. 3 a, what would have been a lid andthermal attach corresponding to lid 110 and thermal attach 116 (namely,lid 410 and thermal attach 416), respectively, of FIG. 1 have beenremoved from the package depicted in FIG. 4 a to allow the relativeposition of other items under the lid to be seen. Also just as withFIGS. 3 a–c, die 430 is substantially centered relative to substrate412, and is attached to substrate 412 via underfill 420 and solder balls422 (also known as C4 bumps, like solder balls 122 of FIG. 1) withunderfill 420 shown protruding from underneath and just beyond the edgesof die 430. Also, solder balls 422 provide electrical connectionsbetween die 430 and substrate 412, which is a printed circuit board withconductors 413 forming electrical connections between solder balls 422and pins 424 (pins 424 being appropriate for a pin grid array package,whereas balls 324 were appropriate to a ball grid array package).

The invention has been described in conjunction with the preferredembodiment. It is evident that numerous alternatives, modifications,variations and uses will be apparent to those skilled in the art inlight of the foregoing description. It will be understood by thoseskilled in the art, that the present invention may be practiced insupport of other combinations of functions in a computer system.

The example embodiments of the present invention are described in thecontext of ball grid array and pin grid array packages carrying a die onwhich is disposed electronic circuitry. However, the present inventionis applicable to a variety of package types and to a variety ofelectronic, microelectronic and micromechanical devices. Although thepresent invention is described in the context of packages attached torigid printed circuit boards that are commonly in use, the presentinvention is also applicable to packages being attached to sheets offlexible material or other surfaces providing electrical connections.Also, although the present invention is described in the context ofpackages with the sealant be disposed in a substantially rectangularpattern, the present invention is applicable to other patterns in whichthe sealant may disposed, regardless of whether the pattern in which thesealant is disposed is influenced by the shape of the lid and/or thesubstrate, or not. Indeed, the sealant may be disposed in circular orother polygonal patterns, or the sealant may be disposed as a pluralityof spots rather than line segments. Furthermore, although the presentinvention is described in the context of packages that enclose a singledie on which electronic circuitry is disposed, the present invention isapplicable to packages enclosing multiple separate dies, and/or diescomprised of smaller dies.

1. A package, comprising: a substrate with an inner surface to which adie is to be attached, forming electrical connections through thesubstrate, between the die and the exterior of the package; a lid withan inner surface facing the inner surface of the substrate; thermalattach disposed between the die and the inner surface of the lid; and alayer of sealant disposed between and contacting the substrate and thelid to bond the lid to the substrate, wherein the sealant is disposed ina pattern with at least one break in the pattern, with the at least onebreak in the pattern remaining subsequent to the substrate and lid beingassembled together.
 2. The package of claim 1, wherein the package is aball grid array package.
 3. The package of claim 1, wherein the packageis a pin grid array package.
 4. The package of claim 1, wherein avent-hole is formed through the lid.
 5. The package of claim 1, whereinthe pattern in which the sealant is disposed between the lid and thesubstrate is a substantially rectangular pattern with at least onebreak.
 6. The package of claim 5, wherein the rectangular pattern hasfour breaks, one in each side of the substantially rectangular pattern.7. The package of claim 6, wherein the four breaks comprise a minimum of10% of the total length of what would otherwise be an unbrokensubstantially rectangular pattern.
 8. The package of claim 5, whereinthe rectangular pattern has four breaks, one in each corner of thesubstantially rectangular pattern.
 9. The package of claim 8, whereinthe four breaks comprise a minimum of 10% of the total length of whatwould otherwise be an unbroken substantially rectangular pattern. 10.The package of claim 1, wherein the substrate is susceptible toabsorbing moisture, and the pressure existing between the substrate andthe lid is as a result of moisture being released within the package bythe substrate and being converted to steam.
 11. The package of claim 10,wherein the substrate is comprised of organic material.
 12. The packageof claim 1, wherein the die is attached to the substrate using acontrolled collapsed chip connection.
 13. An apparatus, comprising: asubstrate with an inner surface; a lid with an inner surface facing theinner surface of the substrate; a die on which electronic circuitry isdisposed, enclosed between the substrate and the lid, and attached tothe inner surface of the substrate which provides electrical connectionsbetween the die and the exterior of the package; thermal attach disposedbetween the die and inner surface of the lid; and a layer of sealantdisposed between and contacting the substrate and the lid to bond thelid to the substrate, wherein the sealant is disposed in a pattern withat least one break in the pattern, with the at least one break in thepattern remaining subsequent to the substrate and lid being assembledtogether.
 14. The apparatus of claim 13, wherein the pattern in whichthe sealant is disposed between the lid and the substrate is asubstantially rectangular pattern with at least one break.
 15. Theapparatus of claim 14, wherein the rectangular pattern has four breaks,one in each side of the substantially rectangular pattern.
 16. Theapparatus of claim 13, wherein the die is attached to the substrateusing a controlled collapsed chip connection.